Preheat oven to 350°F (175°C) and line a 12-cup muffin tin.
Whisk flour, cocoa powder, sugar, baking soda, baking powder, and salt together.
Add egg, buttermilk, oil, and vanilla; mix until smooth.
Gradually pour in hot brewed coffee and mix until combined.
Pour batter into liners and bake 18–20 minutes; cool completely.
For frosting, dissolve espresso powder in vanilla extract.
Beat butter until creamy, then gradually add powdered sugar and cream.
Add espresso mixture and salt; beat until fluffy.
Frost cooled cupcakes and decorate as desired.